As light propagates through any non-ideal medium, the light experiences dispersion. The dispersion may have many contributing forms that add to the total dispersion, for example polarization mode dispersion or chromatic dispersion, but all forms of dispersion will result in a broadening of a pulse of light. Optical system designers must consider dispersion when designing an optical network. If the amount of dispersion experienced by an optical signal will exceed an acceptable level, then the dispersion will have to be compensated for in some way. Therefore, it is advantageous if the amount of dispersion present in an optical signal can be determined.
Some conventional dispersion compensation systems do not determine the dispersion per se, but instead utilize a bit error rate (BER) monitor that determines a BER by adjusting a tunable dispersion compensation module and monitoring what effect the tunable dispersion compensation module has on the BER. When the BER is minimized it is assumed that the dispersion has been ideally compensated for, and thus the dispersion is assumed to be equal to the dispersion compensation value that resulted in a minimum BER. However, such systems are dependent on the modulation format of the optical signal. For example, a non-return to zero (NRZ) optical signal with a dispersion of 100 ps may have the same BER as a return to zero (RZ) optical signal with a dispersion of 75 ps. Furthermore, these conventional systems assume that the BER and any change in the BER is due to dispersion and a change in dispersion respectively, whereas in reality the BER is a function of many other factors, including insertion losses and noise. In addition, determining a BER requires the demodulation and decoding of an optical signal, which greatly increases the complexity and cost of these dispersion determining systems.